A number of non-traditional machining processes have been developed to provide alternative methods of preparing complex workpieces. Such processes are often employed in the working of castings, forged parts, composite and ceramic parts, and as a finishing step on workpieces where rough machining has been performed using more conventional techniques.
One such technique is electrical discharge machining (EDM). EDM allows removal of metal from a workpiece by the energy of an electric spark arcing between a tool and a surface of the workpiece. During use, both the tool and the workpiece are immersed in a dielectric fluid such as oil. Rapid pulses of electricity are then delivered to the tool, causing sparks to jump or arc between the tool and the workpiece. The heat from each spark causes a small portion of metal on the workpiece to melt, removing it from the workpiece. As the metal is thus removed, it is cooled and flushed away by circulation of the dielectric fluid.
EDM can generally be used to form complex and intricate shapes in a workpiece. However, EDM suffers from a number of limitations. First, the workpiece must be electrically conductive in order to close the electrical circuit necessary to create a spark between the workpiece and the tool. Thus, EDM is not suitable for use on workpieces made of many materials, such as most ceramics or polymers. Second, it can be difficult to achieve the desired final finish to the surface of a workpiece using EDM, and surfaces subjected to EDM typically have an “orange peel” or “sand blasted” appearance. For example, it may be desired to have a final surface finish as rough as 0.8 μm Root Mean Square RMS, or have a smoother mirror finish at approximately 0.02 μm RMS. EDM typically yields, at best, a surface finish between 0.8 and 3.2 μm RMS. Thus, while EDM can be useful for providing a rougher finish, it is generally not suitable for providing highly polished workpieces.
Another non-traditional machining process that tends to provide a smoother finish is ultrasonic polishing, also known as ultrasonic impact grinding. Ultrasonic polishing generally involves the removal of a thin layer of material (e.g. up to 50 μm thick or less) to finish a workpiece to the desired dimensions. The polishing involves the removal of waviness on the surface of the workpiece, typically by selective removal of undesired semi-fine details (e.g. the top portion of long amplitude waveform features present on the surface or the workpiece) and undesired fine details or surface roughness (e.g. the top portion of short amplitude waveform features present on the surface of the workpiece) while leaving desired surface features intact.
Polishing of the workpiece is effected by rapid and forceful agitation of fine abrasive particles suspended in slurry located between the surface of the workpiece and the face of a tool. In order to agitate the abrasive particles in the slurry, during operation the tool is vibrated at frequencies that are generally between 15,000 Hz and 40,000 Hz, although it is possible to use much higher or lower frequencies according to the needs of a particular application.
Various techniques can be used to effect vibration of the tool. One method is to use a magneto-restrictive actuator, where a magnetic field is cyclically applied to a ferromagnetic core. Application of the field causes an effect known as magnetorestriction, whereby the core length changes slightly in response to fluctuations in the magnetic field intensity. Another method to effect vibration uses a piezoelectric transducer that oscillates in response to the application of an electric field, as is known in the art. The transducer is then typically connected to a horn or concentrator having a tool at the working end thereof. The horn increases the amplitude of the oscillation of the tool relative to the oscillation of the actuator or transducer. The horn typically has a generally frustoconical shape, with the tool connected to the narrower working end and the actuator or transducer affixed at the wider or larger end.
During operation, the magneto-restrictive actuator causes the tool to oscillate in a direction generally parallel to the longitudinal axis of the horn, which is typically normal to the surface of the workpiece. During any single cycle, the tool moves from its uppermost position P1 furthest away from the surface of the workpiece (where the tool is at rest) through a mean position P2 (where the tool is moving the fastest) to the lowest position P3 closest to the surface of the workpiece (where the tool is at rest again). As the cycle continues, the tool moves back through the mean position P2 to the uppermost position P1, and so on. In some embodiments, and depending on the configuration of a particular ultrasonic polishing apparatus, the amplitude of oscillation of the tool from P1 to P3 is between 13 and 62 μm, although it is possible to use much higher or lower amplitudes according to the needs of a particular application.
The interaction between the face of the tool, the workpiece and the abrasive slurry depends on the sizing relationship between the abrasive particles in the slurry and the distance between the workpiece and the tool face during the cycle. When the abrasive particles are sized such that they are large enough to be contacted by the tool at the mean position P2, the abrasive tends to be impacted when the tool is moving at its highest velocity. Thus, a greater amount of momentum will generally be transferred to the particles. Where abrasive particles are smaller in size, however, they will be impacted when the tool is closer to the surface of the workpiece (between P2 and P3) and thus moving at a slower velocity. Thus, smaller abrasive particles will generally receive a lesser amount of momentum from the tool. Similarly, where the abrasive particles are larger in size, they tend to be impacted by the tool before it has reached its maximum velocity (between P1 and P2). Thus, there is typically an effective range of abrasive particles sizes (or grit sizes) that work for any particular tool and workpiece combination based on the gap between the workpiece and the tool.
During operation, when the tool impacts any particular abrasive particle, that particle will be forced against the workpiece by the action of the tool. This causes impact stresses on the surface of both the workpiece and the tool. These impact stresses occasionally cause one or more abrasive particles to become fractured, which tends to decrease the size of the particles and is one reason that it is desirable to introduce fresh abrasive particles into the slurry to ensure that the desired abrasive size is retained to ensure the rate of polishing is maintained. Introducing fresh slurry also assists with flushing of the workpiece debris away from the gap between the tool and the workpiece.
The vibrating tool thus effectively acts as a hammer that periodically strikes the abrasive particles and chips out small portions of the workpiece. Material is removed from the workpiece by three main modes: (a) ballistic or cavitation effects causing the abrasive particles to impact the surface of the workpiece, (b) mechanical effects caused by abrasive particles flowing back and forth generally parallel to the workpiece surface (caused by the movement of the slurry), and (c) mechanical effects caused by particles vibrating over the surface of the workpiece or by a buildup of abrasive particles which crush the surface of the workpiece by bridging the gap between the workpiece and the tool.
One of the major benefits of ultrasonic polishing over EDM is that ultrasonic polishing is non-thermal, non-chemical, and non-electrical. Thus, ultrasonic polishing neither requires nor creates any changes in the metallurgical, chemical or physical properties of the workpiece being polished, other than the removal of material. Ultrasonic polishing can therefore be used to shape many different types of materials, including hard materials and materials that are not electrically conductive, such as ceramics and glass, which cannot generally be shaped using EDM.
Ultrasonic polishing can also be performed without the need for the dielectric fluid required in EDM. In many cases, a simple slurry mixture of abrasive particles in water, oil or an emulsion is all that is required.
Ultrasonic polishing can also result in much smoother surface characteristics to the finished workpiece. With a proper selection of abrasive, frequency of oscillation, amplitude of oscillation, tool, and spacing between the tool and the workpiece, ultrasonic polishing can result in surfaces with mirror finishes (less than 0.25 μm RMS).
However, ultrasonic polishing also faces a number of challenges. Polishing is typically much slower than many other material removal techniques, such as EDM. Thus, it can take much longer to obtain a desired final surface. Furthermore, the tool used in ultrasonic polishing is generally made of a material that is generally softer than the workpiece. This can result in very high rates of wear to the tool in comparison to the rate of material removal from the workpiece, which can make it difficult to maintain an accurate tool shape to ensure that the workpiece receives the desired profile. As a result, it is often necessary to change tools after polishing of a single workpiece, or even use multiple tools during polishing of the same workpiece. Tools that have been worn down are often simply discarded, which can be expensive and wasteful.
Accordingly, there is a need for an improved method and apparatus for preparing workpieces having smooth, polished surfaces.